Method and apparatus for the recovery of heat and chemicals from black liquor



Patented Oct. 7, 1941 METHOD AND APPARATUS FOR THE RECOV- ERY OF HEATAND CHEMICALS FROM BLACK LIQUOR Vincent P. Owens, Plainfield, N. J.,assignor to Combustion Engineering Company, Inc., New York, N. Y., acorporation of Delaware Application October 14, 1939, Serial No. 299,517

3 Claims.

This invention relates to a self-sustaining method of and apparatus forthe recovery of heat and chemicals from black liquor obtained in thepreparation of pulp from wood by the caustic soda or sulphate processand in general includes an evaporator for removing water from the blackliquor to increase its density, a recovery furnace for the recovery ofchemicals and a waste heat boiler associated with the furnace.

The p-rimary object of my invention is the provision of improvementswhereby the recovery unit is maintained self-sustaining despite varyingoperating conditions.

How the foregoing, together with such other objects and advantages aswill hereinafter appear or are incident to my invention, are realized,is illustrated in preferred form in the accompanying drawing, whereinthe figure is a more or less diagrammatic sectional elevational View ofa recovery system embodying my invention.

'Referring to the drawing black liquor received from a suitable source,such as multiple effect evaporators customarily employed in associationwith recovery units, is led by means of a pipe 2 to a head or storagetank 3, the liquor thus in 'r troduced normally having a given densityof approximately 45% of solids. The liquor is led from the tank 3 bymeans of a pipe 4 to an evaporator 5 where the evaporation of watertakes place by virtue of hot gases led thereinto as by means of aconduit or connection 6 leading from the oiltake of the waste heatboiler 8 to the inlet 9 of the evaporator 5.

High density liquor under pressure of a pump 5a and heated by a heater5b is led from the evaporator to the nozzles l of the recovery furnaceby means of pipes |2, this high density liquor being sprayed into thefurnace by means of the nozzles, the liquor being fed to thenozzles at apressure of at least substantially 50 pounds and at a temperature of atleast substantially 230, as disclosed in the application of Fay HarryRosencrants and Alexander Leopold Hamm, Serial No. 193,398, led March 2,1938.

The furnace has its two side walls |3 (only one of which appears in thedrawing) and its front wall i4 lined with closely spaced upright streamevaporating tubes i5 connected at their lower ends into bottom headersI6 and at their upper ends into upper headers which are con- "l nectedinto the circulation of the boiler 8 as by means of upcomers I8. Therear wall I9 of the furnace is lined with similar closely spaced uprightsteam evaporating tubes 2E) connected at,

their lower ends into a lower header I6 yand at their upper ends intothe bottom drum 2| of the boiler 8.

` Preheated air in amounts sufficient for combustion is introduced intothe furnace as by means of a plurality of air inlets 22. The furnace isprovided with a refractory lined bottom 23 and the recovered chemicalsmay be withdrawn through a spout 24.

The waste heat boiler 8 above referred to comprises in general the lowerdrum 2| above mentioned, an upper drum 25, a front bank of uprightboiler tubes 2B connecting the drums 2| and 25, a rear bank of uprightboiler tubes 21 connecting said drums, a plurality of widely spacedup-right boiler tubes 28 connecting the lower drum 2| with an upper drum29 spaced horizontally from the upper drum 25, and circulation tubes 39and 3| connecting the upper drums 25 and 29. A superheater 32 is locatedin the space between the bank of boiler tubes andthe boiler tubes 28. t

A baffle 33 located at the front portion of the front bank of boilertubes 2S extends upwardly from the lower drum 2| to a point somewhatbelow the upper drum 25 and a baffle 34 located at the rear portion ofsaid front bank of boiler tubes extends downwardly from the upper drum25 to a point somewhat above the lower drum 2|. Laterally extendingbaille plates 35 extend from the baille 3d to the front tubes of therear bank of boiler tubes 2l and a rear bai-lle 35 is provided at therear of said rear bank of boiler tubes. This rear baille terminates atits upper end at a point below the upper sheathing wall 3l, thusproviding the offtake 1 above referred to. Rear sheathing 33 is providedto form the conduit or connection 6 leading from the oitake 'i to theinlet 9 of the evaporator 5.

I he baling referred to provides a plurality of passages for the gases,it being noted that the hot gases enter at the top of the fro-nt bankfor downward flow thereover, and then pass in upward flow over the rearbank for exit through the oiftake l from whence they are led through theconduit il and inlet 9 into the evaporator.

The temperature of the waste gases entering the evaporator after passingthrough the gas passages in normal ilow and temperature is such thatliquor introduced into the evaporator at a given density of of solidswill be increased in the evaporator to a density of approximately to'70% of solids for introduction into the furnace so as to maintain adrying and burning rate having a ratio such that the unit will beselisustaining, as will further appear.

In order to prevent the furnace from becoming wet and inoperative in theevent that the waste liquor being supplied to the evaporator drops belowthe aforesaid given density of 45% of solids, as for example, would bethe case if there was an unbalancing of the multiple effect evaporatorsusually associated with the system, I have provided means for supplyinggases of higher temperature to the evaporator whereby this lower densityliquor is increased to the high value above mentioned which is necessaryfor self-sustaining operation of the unit while at the same timeavoiding operating difficulties in the steam generating equipment, aswill further appear.

This temperature control means comprises a bypass opening 39 provided inthe upper portion of the baille 34 and an adjustable damper 40controlling said opening. When the damper lil] is closed as shown in thedrawing the hot gases enter at the top of the rst pass and flowdownwardly over the tubes of the rst bank 2S, then pass to the bottom ofthe second pass and ow upwardly over the tubes of the second bank 2l andfinally iiow through the upper oiftalre 1 into the conduit 6 where theyflow downwardly for introduction into the evaporator 5 through theevaporator gas inlet 9. An opening 4I is provided at the lower portionof the rear baffle 35 through which some of the waste gases may flowinto the lower portion.of the conduit 6, as indicated by the arrow 42.

In normal operation when the black liquor introduced into the evaporatoris of a given density of approximately 45% of solids the damper 40 is inits closed position to provide gas flow as just described and with thiscondition of liquor introduction and waste gas flow the furnace will beself-sustaining.

If for any reason the density of the liquor being introduced into theevaporator should drop to a lower density than said given density and ifthe waste gas now to the evaporator should remain as just described,then the density of the liquor leaving the evaporator would also belowered. This lower density liquor when introduced into the furnacebecause of its carrying larger quantities of water than would permit ofself-sustaining operation of the furnace, would require additional heatin the furnace to evaporate the additional water and to obtain this heatauxiliary fuel would have to be introduced into the furnace, and thiswould materially increase the cost of operation. If the heat were notadded the furnace would become wet and inoperative.

In order to control the evaporation in the evaporator I have providedthe aforesaid bypass means whereby the temperature of the gases enteringthe evaporator may be increased, it being noted that when the damper 40is opened boiler heating surface is short circuited and hot enteringgases pass directly through the bypass 39 across the upper portion ofthe boiler tubes to and through the offtake 1 and into the conduit 6leading to the evaporator. Thus an instantaneous increase in thetemperature of the gases entering the evaporator is effected to give therequired control which will keep the furnace supplied with liquor of thedensity necessary for self -sustaining operation.

The system may thus be operated continuously as a self-sustaining onebecause by the control afforded through the medium of the adjustablebypass means I obtain a flexibility of control to raise and lower thetemperature of gases to the evaporator which enables me to maintain asupply of black liquor to the furnace of constant density even thoughthe density of the liquor supplied to the evaporator varies.

The importance of the foregoing will be seen from the following. Whenthe highly concentrated heated black liquor leaves the nozzles furtherconcentration takes place due to the reduction in pressure which resultsin the flashing of water content of the black liquor into steam. Due tothis and to the fact that the black liquor is delivered to the nozzlesat an elevated temperature the gravitating particles of black liquor aredried -to substantial dryness before lodging and collecting on thebottom to a suflicient degree to sustain combustion there without theaid of other fuel. There the solids are brought to incandescence in areducing atmosphere (insufficient air for complete combustion beingpassed through the bed of particles as by means of preheated air inlets22a), chemical is converted for recovery and flows out in molten formand the volatile combustibles rise and are consumed in the combustionspace above the bed furnishing the additional heat required for dryingthe gravitating black liquor particles. The installation is thusself-sustaining when the liquor supplied to the evaporator is of normaldensity despite the fact that the combustion of the combustiblevolatiles and the drying of the gravitating black liquor particles isdone in a space defined by steam evaporating tubes subject to radiantheat. In order to maintain the furnace self-sustaining under varyingconditions occurring in the black liquor treating portion of the systemwhich would result in the liquor supplied to the evaporator being belownormal density, it is absolutely necessary to meet such conditions, andI do this by means of the control above described. It will also be seenthat I am enabled to do this without incurring operating difficulties inthe steam generating system because the slagging tubes and superheatertubes abstract sufficient heat from the gases to cause condensation ofthe major portion of the volatilized chemicals and precipitation of thesame in solid form. Due to agglomeration the solid particles reach asize suicient to gravitate into the combustion chamber proper directlytherebelow.

Thus the system operates to minimize carry over into the convectionbank, thus preventing clogging of the tubes of the convection bank indirectly by-passing hot gases from the waste gas space to theevaporator, to obtain superheated steam, to reduce slaggingdifficulties, to return a maximum of recoverable chemical to thecombustion chamber and to be self-sustaining despite varying operatingconditions.

I claim:

1. The self-sustaining method of recovering chemicals from black liquorwhich comprises normally supplying liquor of given low density to awaste heat evaporator; increasing the density of said black liquor insaid evaporator to a given high density by normally delivering wastegases from a recovery furnace and boiler unit into said evaporator aftersaid waste gases have passed over and among the boiler surfaces of saidunit; delivering said high density liquor into said recovery furnace andboiler unit at a given rate; and maintaining the density of the liquorleaving said evaporator and being delivered into said furnace and boilerunit constant when the liquor supplied to the evaporator drops belowsaid given low density by controllably by-passing hotfwaste gases insaid boiler and furnace unit to increase the temperature of the wastegases delivered into said evaporator.

2. The self-sustaining method of recovering chemicals from black liquorwhich comprises normally supplying black liquor at a given density ofapproximately 45% of solids to a waste heat evaporator; normallydelivering waste gases from a recovery furnace and boiler unit afterthey have passed over and among the boiler surfaces of said unit intosaid evaporator at a temperature to raise the density of said liquor toapproximately 65 to 70% of solids; delivering said high density liquorinto said recovery furnace and boiler unit at a given rate whereby thefurnace will be self-sustaining; and maintaining the density of theliquor leaving said evaporator and being delivered into the furnaceconstant at the aforesaid high density when the liquor supplied to theevaporator drops below said given low density' by controllablyby-passing hot waste gases in said furnace and boiler unit to increasethe temperature of the waste gases delivered into said evaporator.

3. The self-sustaining method of recovering chemicals from black liquorwhich comprises normally supplying black liquor of given low density toa waste heat evaporator; increasing the density of said black liquor insaid evaporator to a given high density by normally deliv-V ering Wastegases from a recovery furnace and boiler unit into said evaporator aftersaid waste gases have passed over and among the boiler surfaces of saidunit; delivering said high density liquor into said recovery furnace andboiler unit at a given rate; chilling slag forming particles in thegases in advance of said boiler; and maintaining the density of theliquor leaving said evaporator and being delivered into said furnace andboiler unit constant when the liquor supplied to the evaporator dropsbelow said given low density by controllably by-passing hot Waste gasesVin said furnace and boiler unit to increase

